Recent advances in phenoxyl radical complexes of salen-type ligands as mixed-valent galactose oxidase models

Abstract

The interplay between redox-active transition metal ions and redox-active ligands in metalloenzyme sites is an area of considerable research interest. Galactose oxidase (GO) is the archetypical example, catalyzing the aerobic oxidation of primary alcohols to aldehydes via two one-electron cofactors: a copper atom and a cysteine-modified tyrosine residue. The electronic structure of the oxidized form of the enzyme (GO(ox)) has been investigated extensively through small molecule analogues including metal-salen phenoxyl radical complexes. Similar to GO(ox), one-electron oxidized metal-salen complexes are mixed-valent species, in which molecular orbitals (MOs) with predominantly phenolate and phenoxyl π-character act as redox-active centers bridged by mixing with metal d-orbitals. A detailed evaluation of the electronic distribution in these odd electron species using a variety of spectroscopic, electrochemical, and theoretical techniques has led to keen insights into the electronic structure of GO(ox).

Figure 1

Schemes of Marcus-Hush coupling in (a) symmetric and (b) non-symmetric mixed-valent complexes.

Figure 2

(left) TD-DFT β-HOMO → β-LUMO transition for [Ni11]⁺ and (right) spin density plot for [Ni11]⁺ showing radical localization on the amino-phenolate [35].

Figure 3

X-ray structure of Cr-tacn phenoxyl radical complex [42].

Figure 4

Metrical parameters of (a) neutral and (b) oxidized Cr-tacn phenolate complex [42].

Figure 5

X-ray structure of [Cu9]⁺SbF₆⁻ [45].

Figure 6

X-ray structure of [Cu3]⁺SbF₆⁻ [46].

Figure 7

Solid state Cu K-edge of Cu3 (black) and [Cu3]⁺SbF₆⁻ (red) inset: Cu pre-edge region (1s → 3d transition) [46].

Figure 8

Solid state sulfur K-edge of neutral (dotted) and oxidized (solid) complexes Cu12^SMe2 and Cu12^SiPr2[19] .

Figure 9

(a) UV-Vis spectra of [Cu9]⁺ (blue), [Cu12^SiPr2]⁺ (red), and [Cu14^SiPr,OMe]⁺ (black); (b) UV-Vis spectra of [Cu9]²⁺ (blue), [Cu12^SiPr2]²⁺ (red), and [Cu14^SiPrOMe]²⁺ (black). Conditions; 0.1mM complex in CH₂Cl₂ at 298 K [20].

Figure 10

UV-Vis-NIR spectra of Ni3 (black), [Ni3]⁺ (red), and [Ni3]⁺(py)₂ (blue). Conditions, 0.08 mM complex in CH₂Cl₂ at 298 K. [Ni3]⁺(py)₂ in 1:1 CH₂Cl₂:pyridine [40].

Figure 11

UV-Vis-NIR spectra of neutral (black) and one-electron oxidized (red) group 10 metal-salen series: (a) Ni, (b) Pd, (c) Pt. Conditions; 0.1 mM complex, CH₂Cl₂ at 298K [43].

Figure 12

UV-Vis-NIR of Ni11 (solid) and [Ni11]⁺ (dashed). Conditions, 0.08 mM complex in CH₂Cl₂ at 298 K [35].

Figure 13

X-band EPR spectra (solid: data; dashed: simulations) of (a) 1 mM [Ni3]⁺ , (b) 1 mM [Ni3]⁺ + 0.1 M tetrabutylammonium perchlorate, (c) 1 mM [Ni3]⁺ + 2 eq. pyridine. Conditions, 1 mM complex in frozen CH₂Cl₂ at 77 K [40].

Figure 14

X-band EPR spectra of (a) [Cu10]⁺ and (b) [Cu10H]²⁺. Conditions, 1 mM complex and 0.1 M tetrabutylammonium perchlorate in frozen CH₂Cl₂ at 77 K [54].

Figure 15

Resonance Raman (rR) spectra of GO_ox (λ_ex = 875 nm) [55].

Figure 16

Resonance Raman (rR) spectra of (a) Cu3, and (b) [Cu3]⁺ at 298 K; rR spectra of (c) Cu4 and (d) [Cu4]⁺ at 213 K (λ_ex = 413 nm). Conditions, 1 mM complex in CH₂Cl₂ [46].

Figure 17

Resonance Raman of Ni11 (solid) and [Ni11]⁺ (dashed) in CH₂Cl₂ at 213 K (λ_ex = 413 nm). Solvent = • Conditions, 1 mM in CH₂Cl₂ at 213 K [35].

Scheme 1

Consensus mechanism of GO.

Scheme 2

Generic salen condensation reaction.

Scheme 3

Salen ligands.

Scheme 4

Synthesis of non-symmetric salen ligands.

Scheme 5

Synthesis of “half-reduced” salen ligands.